| they'll get substituted into other properties at
| computed-value time, when it's already too late to actually process
| them (they're processed in the transition from cascaded to specified
| value).
This is why I changed the wording of my spec to say that reference
resolution happens just after cascaded time. If you do so, the 'inherit'
keyword get replaced just before the value computation and, actually, it
will work as expetected.
See http://fremycompany.com/TR/2012/ED-css-custom/#value-resolution